1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic system, in particular to an image forming apparatus such as a copying machine, a printer or a facsimile.
2. Related Art
For example, in an image forming apparatus such as a digital copying machine or a printer using an electrophotographic system, a one-component magnetic non-contact developing method is often used in which a maintenance interval is long and also a structure is relatively simple. This developing method is one in which load given to a photosensitive drum that is an image bearing member and a driving system by performing non-contact developing is little and which is suitable for high reliability, but a charge amount of a toner that is a developer is relatively low. Thus, in order to charge a developing image developed on the photosensitive drum, that is, a toner image after developing, the developing method is often combined with pre-transfer charging.
In general, a reversal developing system is often used as a developing system. In the reversal developing system, image exposure corresponding to image information to be printed is conducted to a uniformly charged photosensitive drum surface by using a laser, an LED or the like, to thereby form an electrostatic latent image, and then, the portion where a potential thereof is lowered by having undergone the exposure is made to be adhered with the toner charged with the same polarity as that of the photosensitive drum, to thereby form a toner image.
As a method of transferring the toner on the photosensitive drum, which has been subjected to the reversal developing, onto a transferring material, and separating it, corona transfer and electrostatic separation utilizing a corona charging device is often used. In this method, charging and charge-removal can be conducted while a charging means is in non-contact with the photosensitive drum or the transferring material, and thus, there is little load to a charging member and the photosensitive drum. Therefore, this method provides high reliability.
However, the transferring material such as paper used in the image forming apparatus of the reversal developing system is not always recommended paper designated by a maker, and includes recycled paper, intermediate sheet, special paper and the like. The image forming apparatus is strongly influenced particularly in separating property of the transferring material by a type of the transferring material, temperature and humidity of the apparatus main body, and an image forming mode.
Generally, in the reversal developing system, a background portion of the photosensitive drum surface, that is, the portion to which the toner is not adhered does not undergo exposure, and thus, is still in a high potential state after the toner image formation. Then, in this state, pre-transfer charging is conducted to the toner developed on the photosensitive drum in order to stably improve the toner charge amount. Thus, a surface potential of the photosensitive drum also rises.
Further, in the next transfer process, when the back surface of the transferring material such as a sheet of paper is imparted with a charge with opposite polarity to that of the toner and then the toner is transferred onto the surface of the transferring material, the transferring material is charged with the opposite polarity to that of the toner, that is, the opposite polarity to that of the background portion (non-exposure portion) of the photosensitive drum surface. Thus, a large electrostatic adhesive force is generated between the photosensitive drum and the transferring material, and the subsequent separation of the transferring material from the photosensitive drum becomes difficult. Normally, since a background portion (white ground portion) occupies a larger area than a character portion and the like to which the toner is to be adhered in an image of a document or the like, the electrostatic adhesive force is particularly large, and the separation is difficult.
The above-described problem that occurs in the image forming apparatus is prevented by adjusting charging conditions or transfer conditions in the case where recommended paper designated by a maker which is a high rigidity transferring material is used as a transferring material. However, in the case where a low rigidity transferring material such as recycled paper or intermediate sheet is used, there occurs a problem of malfunction of separation from the photosensitive drum. Further, the image forming apparatus is largely influenced by temperature and humidity of the apparatus main body, and furthermore, various printing methods such as double-sided printing and multiple printing are conducted to the above transferring materials. Therefore, actions such as transfer, separation, fixing and conveyance are imparted to a sheet of transferring material many times so that the characteristics of the transferring material (resistance, surface property, curl and the like) change. Thus, it has been difficult to maintain stable conveying property (at the time of transfer separation) and image property in the apparatus.
The present invention has been made in view of the above, and an object of the present invention is therefore, even in the case where an image is formed on a transferring material having low rigidity, such as recycled paper or intermediate sheet, to provide an image forming apparatus capable of satisfactorily separating such a transferring material from a photosensitive body.
Another object of the present invention is, even in the case where an image is formed on a second surface of a transferring material, to provide an image forming apparatus capable of satisfactorily separating the transferring material from a photosensitive body.
Other objects of the present invention will be apparent from the following detailed description referring to the attached drawings.